Recently, hot-formed ultra-high-strength members have been widely used as automotive structural members for purposes such as improving fuel efficiency by weight reduction and passenger protection, and a great deal of research has been conducted for such purposes.
The invention disclosed in U.S. Pat. No. 6,296,805 B1 may be a typical example thereof. According to the patent, an Al—Si coated steel sheet is heated to 850° C. or greater, and a hot press forming process and a cooling process are performed to form a member having a martensite phase and thus, ultra-high strength, within the range of 1600 MPa or greater in tensile strength. However, according to the patent, a coating layer having aluminum (Al) as a main phase is formed, and thus, when a blank is heated in a heating furnace, the Al coating layer may melt at a temperature equal to or greater than the melting point of the Al coating layer. Thus, aluminum (Al) may be fused with rolls provided in the heating furnace. In addition, when the Al—Si coated steel sheet is heated in the heating furnace, hydrogen from steam present in the air may diffuse into steel, and thus hydrogen delayed fracture may occur in a hot-formed member.
The invention disclosed in Korean Patent Application No. 10-2011-7000520 may be a technique for solving this problem. According to the patent, a coil of an Al coated steel sheet is annealed for removing hydrogen introduced during an Al coating process, preventing an Al coating layer from flowing down in a rapid heating process, and thus preventing the occlusion of hydrogen. However, according to the patent, a dew point of −10° C. or higher is required in the coil annealing process to prevent the occlusion of hydrogen, and a batch annealing process is performed in air atmosphere to prevent surface quality deterioration of a steel sheet. Therefore, large amounts of oxides may be formed on the surface of a steel sheet, and the spot weldability of a hot-formed member may deteriorate.
In addition, Korean Patent Application No. 10-2010-7019003 discloses a technique in which a batch annealing process is performed to partially alloy an Al—Fe coating layer and reduce a hot forming process time. However, according to the disclosed technique, when an Al—Fe coated coil is passed through a roll leveller to obtain flatness, a partially alloyed portion may be separated due to stress.
Therefore, it is required to develop an Al—Fe alloy coated steel sheet for hot forming, which is capable of guaranteeing the spot weldability of a hot-formed member by suppressing the formation of surface oxides after an Al—Si coated coil is batch annealed, preventing coating layer separation when an Al—Fe coated coil passes through a roll leveller after the batch annealing process, and suppressing the occlusion of hydrogen to guarantee high hydrogen delayed fracture resistance after a hot forming process in air or another atmosphere.